With recent reduction in size of an electronic device, demands for a high-capacity secondary battery are increasing. Particularly, a lithium ion secondary battery having a high energy density and excellent large-current charge/discharge characteristics compared with a nickel•cadmium battery or a nickel•hydrogen battery is attracting attention. A high-capacity lithium ion secondary battery has been heretofore widely studied, but in recent years, high performance is also increasingly required of the lithium ion secondary battery, and it is demanded to further raise the capacity.
The negative electrode material used for such a lithium ion secondary battery is in many cases a graphite material or an amorphous carbon in view of cost and durability. As the method for raising the capacity of the lithium ion secondary battery, it is designed to increase the electrode density and pack a charge/discharge active material as much as possible in the limited battery volume.
A protective film called SEI (Solid Electrolyte Interface) is usually formed on the surface of the carbon material negative electrode by a reaction with an electrolytic solution, whereby chemical stability of the negative electrode is maintained. However, production of the SEI film is associated with generation of a gas as a side-reaction product. Particularly, in the case of a prismatic battery, the thickness of a battery case is expanded due to the gas generation. In anticipation of this battery swelling, the battery is designed by previously securing the volume for the generated gas portion and therefore, the limited battery volume cannot be effectively utilized as a volume for packing an active material.
Also, when the battery is stored at high temperatures, deterioration of the SEI film is liable to occur, and the negative electrode and the electrolytic solution react in the deteriorated site, giving rise to a problem that a larger amount of gas is generated. The gas generation amount is greatly governed by the stability of the SEI film and therefore, many studies to form a good SEI film are being made.
In view of mechanism that the SEI film is formed by a reaction of a negative electrode active material with an electrolytic solution, the film is highly subject to effects of the amount of an oxygen-containing functional group such as carboxyl group and carbonyl group on the carbon particle surface or the surface structure of a carbon material. For forming a good SEI film, it has been proposed to perform surface modification of the carbon material.
For example, Patent Document 1 discloses a carbon material where the content of an acidic functional group is 5 m-equivalent/kg or less and 0.3 μmol/m2 or more. In this technique, the amount of an acidic functional group present on the carbon material surface is controlled by applying a physical impact to raw material graphite and therefore, reduction in the irreversible capacity at the initial charge/discharge or enhancement of cycle characteristics may be expected, but the SEI film is not sufficiently stabilized due to the small amount of an acidic functional group, and it must be said that high-temperature storage characteristics required in association with the demand for high capacity are not responded to.
Patent Document 2 discloses a method of performing a treatment with an acidic aqueous solution or an alkali aqueous solution at a temperature of 20 to 100° C. to introduce a surface functional group into the carbon material surface and uniformly form an SEI film. However, Patent Document 2 is silent on improvement for the increase of irreversible capacity ascribable to a reaction of an acidic functional group with an electrolytic solution at the initial charge as well as for the gas generation.
That is, as to the extent to which an acidic functional group should be introduced into the carbon material surface so as to suppress an excessive reaction of an acidic functional group with an electrolytic solution at the initial charge or the introduction amount in which an acidic functional group should be introduced into the carbon material surface so as to reduce the irreversible capacity and suppress the gas generation, is still not answered at present.